It is known in prior art to provide an SCR filter made up of a filter and an SCR catalyst (selective catalytic reduction NOx catalyst) supported on the filter in an exhaust passage of an internal combustion engine. The SCR filter has a capability of reducing NOx in the exhaust gas by using ammonia as reducing agent. The filter has the function of trapping particulate matter (which will be hereinafter referred to as “PM”) in the exhaust gas. In an exhaust gas purification system equipped with such an SCR filter, ammonia serving as reducing agent is supplied to the SCR filter by an ammonia supply device provided in the exhaust passage.
Patent Literature 1 discloses a technology pertaining to a fault diagnosis apparatus for an exhaust gas purification system equipped with an SCR catalyst provided in the exhaust passage. In the technology disclosed in Patent Literature 1, a certain threshold and a correction coefficient for fault diagnosis is set on the basis of intermediate characteristics between purification characteristics in the case where the exhaust gas system is sound and purification characteristics in the case where the exhaust gas purification system is faulty. The correction coefficient thus set is used to correct the NOx removal rate that is calculated using the quantity of NOx flowing out of the SCR catalyst as a parameter. The NOx removal rate after the correction is compared with the threshold to diagnose whether or not the exhaust as purification system is faulty.
Non-Patent Literature 1 teaches that increases in the amount of PM deposited in an SCR filter make the ammonia adsorption amount defined as the amount of ammonia adsorbed in an SCR catalyst supported on the SCR filter more apt to increase.